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The spinal and commissural projections from the superior colliculus in rat and hamster arise from distinct neuronal
Insights
Superior collicular (SC) cells projecting to the spinal cord are distinct from those innervating the contralateral colliculus. A small population of tectospinal cells also collateralize to the thalamus.
Area of Science:
- Neuroscience
- Cell Biology
- Anatomy
Background:
- The superior colliculus (SC) plays a crucial role in sensorimotor integration.
- Understanding the projection patterns of SC neurons is vital for deciphering neural circuitry.
- The existence of dual projections from SC cells to both the spinal cord and contralateral SC has been debated.
Purpose of the Study:
- To investigate whether superior collicular (SC) cells projecting to the spinal cord also innervate the contralateral SC.
- To characterize the morphological differences between tectospinal and SC commissural neurons.
- To identify potential overlap in neuronal populations contributing to different SC pathways.
Main Methods:
- Multiple retrograde labeling techniques using tracers like horseradish peroxidase, True blue, Diamidino yellow, fluorogold, and microspheres.
- Sequential injections of different tracers into the spinal cord, contralateral SC, and thalamus.
- Analysis of double-labeled cells and measurement of neuronal soma size.
Main Results:
- No double-labeled cells were found when labeling tectospinal and SC commissural neurons simultaneously.
- SC commissural neurons were significantly smaller than tectospinal cells (average soma area: 85.4 µm² vs. 214.4–225.9 µm²).
- A small number of double-labeled cells were observed when labeling predorsal bundle and SC, and when labeling thalamus and spinal cord, indicating some collateralization.
Conclusions:
- Tectospinal and SC commissural pathways originate from largely distinct neuronal populations.
- A small subset of neurons projecting to the predorsal bundle possess commissural collaterals.
- Some tectospinal neurons also exhibit collateral projections to the thalamus.
Abstract:
Multiple retrograde labelling techniques were used to determine whether the superior collicular (SC) cells that project to the spinal cord in hamster and rat also innervate the contralateral colliculus. In 25 rats and 13 hamsters, various combinations of the tracers horseradish peroxidase, True blue, Diamidino yellow, fluorogold and rhodamine-labelled latex microspheres were used to label tectospinal and SC commissural neurons. No double-labelled cells were observed in any of these experiments. Analysis of neurons that were retrogradely labelled with horseradish peroxidase showed further that SC commissural neurons were much smaller than tectospinal cells. The average soma area for tectospinal cells in hamster was 225.9 micron 2 and that for such neurons in rat was 214.4 micron 2. The mean soma areas for SC commissural neurons in hamster and rat were 85.4 micron 2, respectively. In additional experiments (6 rats and 6 hamsters), True blue was injected into the left predorsal bundle and Diamidino yellow was deposited into the left SC. In both hamsters and rats, these injections invariably produced a small number of double-labelled cells in the deep layers of the right SC. In a final group of animals (7 rats and 2 hamsters), large thalamic deposits of Diamidino yellow were combined with bilateral injections of True blue into the spinal cord. This combination also produced small numbers of double-labelled neurons in both colliculi. These results indicate that the tectospinal and SC commissural pathways arise from distinct neuronal populations, but that a small number of cells that send axons into the predorsal bundle also have commissural collaterals. They demonstrate further that some tectospinal cells also send axon collaterals to the thalamus.